1. Field of the Invention
This invention relates to an improvement in the manufacture of threaded fasteners that use thermoplastic patches to produce a self-locking effect. More particularly, the invention relates to the application, preferably by spraying, of high-temperature-resistant resin powders (specifically poly (C.sub.6 -C.sub.8 alkyl isophthalamides)) to the threads of heated fasteners, resulting in fusion of the powders to form a locking patch. Such patches retain their self-locking ability at much higher temperatures than was previously possible. Optionally, an adhesive may be pre-applied to the fastener, or mixed with the high-temperature-resistant resin, to facilitate adherence of the resin to the fastener.
2. Description of the Prior Art
The manufacture of self-locking threaded fasteners that use a patch of thermoplastic resin on one side of the threaded fastener, applied by heating the threaded fastener and then spraying powdered resin onto the threads of the fastener, is known in the art and has enjoyed commercial success. U.S. Pat. No. Re. 28,812 (formerly U.S. Pat. No. 3,579,684), for example, describes a method of manufacturing self-locking threaded fasteners comprising the following steps:
1. Application by spraying of a thin, heat-softenable, substantially continuous film of liquid primer or tying material to a selected area of the exterior threads of a fastener such as a bolt. PA1 2. Drying of the primer film. PA1 3. Heating of the primed bolt by high frequency induction, to a temperature at which the bolt will retain sufficient sensible heat to soften and fuse powdered thermoplastic resin. PA1 4. Entraining fine powdered thermoplastic resin, such as nylon 11, in a stream of air and spraying it directly onto one face of the threads of the heated bolt, forming an eccentric locking patch of resin covering part of the threads.
When a bolt prepared in this manner is threadably engaged with a nut, the thermoplastic material adhering to the sides of the thread flanks (the bearing surfaces) provides locking pressure which dramatically increases the torque required for disassembly, thus rendering the bolt self-locking. Patches of this sort can be formed on the internal threads of female fasteners, as taught in U.S. Pat. No. 3,894,509 (Duffy), as well as on the exterior threads of male fasteners.
The prior art describes various methods for enhancing the ease of assembly of such fasteners, such as by applying the resin so as to provide a thicker layer of thermoplastic on the flanks of the threads than on their crests, as in U.S. Pat. No. Re. 28,812. The prior art also discloses methods for increasing the retaining power of such fasteners when the male and female members are at the outside limits of dimensional tolerances by applying the resin so as to form a continuous ridge or bar of plastic above the thread crests, a result accomplished by reducing the application temperature slightly below that required to form a smooth, continuous coating of plastic, as in U.S. Pat. No. 3,787,222. And several different types of heating and spraying machines for applying the thermoplastic resin coatings to the exterior threads of male threaded fasteners have been patented, as in U.S. Pat. Nos. 3,452,714 and 3,530,827. Methods of applying such patches to the internal threads of female threaded fasteners also are known, as illustrated by U.S. Pat. No. 3,894,509.
The use of adhesives as one constituent of coatings for self-locking threaded fasteners also has been suggested. See, for example, U.S. Pat. Nos. 4,282,913 (Trimmer); 3,179,143 (Schultz); 4,632,944 (Thompson); and 4,927,307 (Fitzgerald). None of these references teaches or even suggests use of any adhesive to facilitate adherence of a high-temperature-resistant patch.
Finally, it has been suggested that light weight threaded fasteners can be fabricated entirely of certain thermoplastic resins containing high-modulus fibers for purposes of reinforcement. (See U.S. Pat. No. 4,863,330 to Olez). Among the resins suggested are TORLON.RTM. and XYDAR.RTM.. (Olez at Col. 5, lines 62-68). (TORLON.RTM. is a polyamide/polyimide polymer mixture and XYDAR.RTM. is an aromatic polyester polymer.) But Olez teaches that the bodies of the threaded fasteners themselves, and the threads, may be made from these thermoplastic resins reinforced with high-modulus fibers such as graphite. Olez teaches complex wrapped structures to enhance the strength of such fasteners. There is no suggestion that patches of these resins can be applied to threaded fasteners made of metal, or that once applied as patches to metal fasteners they will exhibit enhanced locking behavior at elevated temperatures. Indeed, Olez does not even suggest that the claimed fasteners fabricated of TORLON.RTM. or XYDAR.RTM. would be self-locking.
None of the prior art products and methods of manufacture for patch-type threaded fasteners solves the problem of retaining self-locking properties at elevated temperatures. Previous powdered patch material has included nylon 11 and similar thermoplastics which are applied to the threads of a fastener to obtain the desired self-locking capabilities. Such thermoplastics retain their self-locking properties only up to temperatures of 275.degree. F. to 400.degree. F. Above those temperatures, self-locking fasteners using conventional powdered thermoplastic resins lose their ability to lock and can no longer pass the test procedures required under qualifying specifications, such as Military Specification Mil-F-18240E, after exposure to elevated temperatures. (This Military Specification specifies a temperature requirement of 250.degree. F., but its test procedures also are commonly used as a benchmark in evaluating the performance of locking fasteners that have been exposed to higher temperatures as well). Prior art users of thermoplastic patches have not been able to produce a patch that combines satisfactory adhesion to metal surfaces, particularly plated surfaces, with acceptable performance after exposure to elevated temperatures--that is, temperatures in excess of 400.degree. F. For example, polyester patches made by the Long-Lok Fasteners Corporation of Cincinnati, Ohio and sold under the trade name "Poly-Lok" are advertised as maintaining effectiveness only up to 400.degree. F. Other conventional plastic patches lose their self-locking ability when exposed to temperatures above about 275.degree. F. to 300.degree. F.
Prior art efforts to solve the problem of retention of self-locking properties at elevated temperatures have focused principally on mechanical approaches. U.S. Pat. No. 3,227,199, for example, discloses threaded fasteners in which a portion of the thread on a male threaded fastener differs in pitch from the remainder of the thread. Assembly of such a male threaded fastener with a female threaded fastener such as a nut creates a jamming action--that is, it retards disengagement of the female threaded fastener from the male threaded fastener by increasing the friction between the engagement surfaces of the two fasteners. As explained in that patent, other solutions to the problem have involved providing two sets of threads of the same pitch inside a single female threaded fastener, with the two sets of threads displaced axially from each other by a small distance in order to create a jamming action; or radially expanding a portion of the threads of a male fastener in order to engage the female fastener more tightly. But all of these solutions depend upon the manufacture of fasteners of non-standard dimensions, which are considerably more expensive and difficult to make than standard fasteners of uniform dimensions.
Accordingly, the object of this invention is to provide self-locking threaded fasteners of standard dimensions that rely on a thermoplastic patch to provide self-locking capability, and that can withstand appreciably higher temperatures than prior art patch-type fasteners while still maintaining the fastener's self-locking capabilities and meeting certain specifications outlining parameters for acceptable self-locking performance.